Starting a single player game with multiplayer options.

What would I have to do if I wanted to start a single player game but have Co-Op things enabled like: When you die, you still have all weapons and items you got before and all monsters you killed are still dead. (Basically set it up like you are playing a 2 player Co-Op game where the 2nd player doesn't do anything.)

Is there a command line for this or is that something you can put into a wad file via WXE or something to make this happen?

Some exes can do this, and there are ways to fool Doom(2).exe into it too.

Discussion here.

Most of the discussion there was about prboom (since it was mainly from the point of view of recording demos), but I presume the same approach should work with other client/server programs (not something I've tested though). Note: prboom runs quite choppily for me in this mode.